heart

Front view of human heart. The pericardium
has been removed, except for the edges around the blood vessels at
the top. The blue arrows show the way blood is brought to the heart
by the vena cavae, passes through
the right atrium and ventricle, and then out through the pulmonary
artery.

Rear view of human heart. The red
arrows show how oxygenated blood arrives at the left atrium, passes
into the left ventricle, and is then pumped out of the heart into
the aorta.

Cross-section of heart, as seen from the
front. This illustration shows the cords or heart-strings
which hold the mitral and tricuspid valves in their correct position.
The semilunar valves are inside the small portions of the aorta and
pulmonary artery which lie within the heart.

Section across the heart to show the valves.
The figure inset shows the level at which the section is made.

A muscular pump that circulates the blood.
An adult human heart weighs between 200 and 425 grams (7 and 15 ounces)
and is slightly larger than a fist. In an average lifetime, a person's heart
may beat more than 3.5 billion times. Each day, the average heart beats
100,000 times, pumping about 7,600 liters (2,000 gallons) of blood.

Location of the heart

The heart is located between the lungs in
the middle of the chest, behind and slightly to the left of the sternum
(breastbone) and in front of the spine. Because the heart is not central,
but lies to the left of the center line, the heart beat is best felt on
the left side of the chest. To make room for it, the left lung is rather
smaller than the left.

Pericardium

A double-layered membrane called the pericardium
surrounds the heart like a sac. The outer layer of the pericardium surrounds
the roots of the heart's major blood vessels and is attached by ligaments
to the spinal column, diaphragm,
and other parts of the body. The inner layer of the pericardium is attached
to the heart muscle. A coating of fluid separates the two layers of membrane,
letting the heart move as it beats, yet still be attached to the body.

Layers of the heart wall

Three layers of tissue form the heart wall. The outer layer of the heart
wall is the epicardium, the middle layer is the myocardium, and the inner
layer is the endocardium.

The walls of the heart are largely made from myocardium, which is a special
kind of muscle tissue. This muscle is so constructed that it is able to
perform the 60 to 70 contractions which the healthy adult human heart undergoes
every minute. On the inside this muscle is provided with a lining of flat
cells called the endocardium, which is direct contact with the blood within
the heart.

Chambers of the heart

The human heart has four chambers. The upper chambers are called the left
and right atria, and the lower chambers are
called the left and right ventricles.
A wall of muscle called the septum separates
the left and right atria and the left and right ventricles.

The two atria are thin-walled chambers that receive blood from the veins:
the right atrium receives deoxygenated blood from systemic veins, while
the left atrium receives oxygenated blood from the pulmonary veins. The
two ventricles are thick-walled chambers that forcefully pump blood out
of the heart. Differences in thickness of the heart chamber walls are due
to variations in the amount of myocardium present, which reflects the amount
of force each chamber is required to generate. The left ventricle is the
largest and strongest chamber. The left ventricle's chamber walls are only
about a half-inch thick, but they have enough force to push blood through
the aortic valve and into the body.

The heart valves

The heart has two types of valves that keep the blood flowing in the correct
direction. The valves between the atria and ventricles are called atrioventricular
valves (also called cuspid valves), while those at the bases of the large
vessels leaving the ventricles are called semilunar
valves.

The right atrioventricular valve is the tricuspid
valve. The left atrioventricular valve is the mitral
valve (also called the bicuspid valve). The valve between the right
ventricle and pulmonary trunk is the pulmonary
valve. The valve between the left ventricle and the aorta
is the aortic valve. Both the pulmonary
and aortic valves are semilunar valves

When the ventricles contract, atrioventricular valves close to prevent blood
from flowing back into the atria. When the ventricles relax, semilunar valves
close to prevent blood from flowing back into the ventricles.

To summarize:

The tricuspid valve regulates blood flow between the right atrium
and right ventricle

The pulmonary valve controls blood flow from the right ventricle into
the pulmonary arteries, which
carry blood to the lungs to pick up oxygen

The mitral valve lets oxygen-rich blood from the lungs pass from the
left atrium into the left ventricle

The aortic valve opens the way for oxygen-rich blood to pass from
the left ventricle into the aorta, the body's largest artery, where
it is delivered to the rest of your body

Pathway of blood through the heart

While it is convenient to describe the flow of blood through the right side
of the heart and then through the left side, it is important to realize
that both atria contract at the same time and both ventricles contract at
the same time. The heart works as two pumps, one on the right and one on
the left, working simultaneously. Blood flows from the right atrium to the
right ventricle, and then is pumped to the lungs to receive oxygen. From
the lungs, the blood flows to the left atrium, then to the left ventricle.
From there it is pumped to the systemic circulation.

Blood supply to the myocardium

The myocardium of the heart wall is a working muscle that needs a continuous
supply of oxygen and nutrients to function with efficiency. For this reason,
cardiac muscle has an extensive network of blood vessels to bring oxygen
to the contracting cells and to remove waste products.

The right and left coronary arteries,
branches of the ascending aorta, supply blood to the walls of the myocardium.
After blood passes through the capillaries in the myocardium, it enters
a system of cardiac (coronary) veins. Most of the cardiac veins drain into
the coronary sinus, which opens into the right atrium.

Conduction system

Image credit: Cleveland Clinic Foundation

An effective cycle for productive pumping of blood requires that the heart
be synchronized accurately. Both atria need to contract simultaneously,
followed by contraction of both ventricles. Specialized cardiac muscle cells
that make up the conduction system of the heart coordinate contraction of
the chambers.

The conduction system includes several components. The first part of the
conduction system is the sinoatrial (SA) node, located in the myocardium
at the top of the right atrium. Without any neural stimulation, the sinoatrial
node rhythmically initiates electrical impulses 70 to 80 times per minute.
Because it establishes the basic rhythm of the heartbeat, it is called the
heart's natural pacemaker. Each electrical impulse from the SA node travels
through the muscle fibers of the atria and ventricles, causing them to contract.
Other parts of the conduction system include the atrioventricular (AV) node,
atrioventricular bundle, bundle branches, and conduction myofibers. All
these components coordinate the contraction and relaxation of the heart
chambers. Although the SA node sends electrical impulses at a certain rate,
your heart rate may still change depending
on physical demands, stress, or hormonal factors.

Cardiac cycle

The cardiac cycle refers to the alternating contraction and relaxation of
the myocardium in the walls of the heart chambers, coordinated by the conduction
system, during one heartbeat. Systole is the contraction phase of the cardiac
cycle, and diastole is the relaxation phase. At a normal heart rate, one
cardiac cycle lasts for 0.8 second.

Heart sounds

The sounds associated with the heartbeat are due to vibrations in the tissues
and blood caused by closure of the valves. Abnormal heart sounds are called
murmurs.

Heart rate

The sinoatrial node, acting alone, produces a constant rhythmic heart
rate. Regulating factors are reliant on the atrioventricular node to
increase or decrease the heart rate to adjust cardiac output to meet the
changing needs of the body. Most changes in the heart rate are mediated
through the cardiac center in the medulla oblongata of the brain. The center
has both sympathetic and parasympathetic components that adjust the heart
rate to meet the changing needs of the body.

Peripheral factors such as emotions, ion concentrations, and body temperature
may affect heart rate. These are usually mediated through the cardiac center.